In dentistry, practitioners use a variety of restorative materials in order to create crowns, veneers, direct fillings, inlays, onlays and splints. Composite resins are a type of restorative material which are suspensions of strengthening agents, such as mineral filler particles, in a polymerizable resin matrix. These materials may be dispersion reinforced or particulate reinforced, depending on the type of filler, or may be hybrid composites or flowable composites, depending on the filler loading. A full discussion of these materials is included in U.S. patent application Ser. No. 09/270,999, entitled "Optimum Particle Sized Hybrid Composite," C. Angeletakis et al., filed on Mar. 17, 1999, now pending (incorporated herein by reference in its entirety). Highly pure submicron particles are useful in these composite resin materials because they impart the desirable optical properties of high gloss and high transparency.
In a composite material, such as a tooth colored dental restorative, the resin matrix and the filler have to be matched in their refractive index to achieve a transparency similar to tooth structure. In addition, this transparency is necessary for the material to be cured using visible light initiation of polymerization. The formulator has relatively wide choices for adjusting the refractive index of the resin because resins in the range of about 1.48 to about 1.55 are easily available commercially. However, glass fillers are much more complex to formulate. The purity of the glass filler must be very high because small amounts of impurities, down to the low ppm level, show very prominently when the filler is dispersed in the resin. Moreover, the refractive index of the polymerized resin must closely match the refractive index of the filler. This is particularly critical with small particle sizes since it is known that light scattering, which is responsible for the opacity in a dental composite, is proportional to the absolute difference between the refractive index of the filler and the refractive index of the resin. If the ground filler dispersed in a resin matrix does not have a transparency value of at least 45, measured as described hereinafter in the Detailed Description of the Invention, then it is difficult to formulate a final restorative material, including pigments and other additives, having the desired shading to match the patient's tooth color.
Milling methods previously used to produce submicron particles have been found unacceptable for filler in dental composites because of the impurities that result from abrasion and spalling. The inclusion of impurities in dental composites can decrease transparency and negatively affect color, making the composites unacceptable for use in dental cavities. Examples of prior art agitator mills are set forth in U.S. Pat. Nos. 5,335,867; 4,129,261; and 4,117,981, all assigned to Draiswerke GmbH and each incorporated herein by reference in its entirety; and 5,065,946, assigned to Matsushita Electric Industrial Co. and incorporated herein by reference in its entirety. These prior art mills typically include ceramic or metallic agitators and grinding chambers. During milling, the ceramic or metallic material of the agitator and grinding chamber spalls and abrades, and the abraded particles become intimately mixed with the material being ground. In the case of fillers for dental restoratives, these abraded particles are unacceptable due to their negative impact on the optical properties of the restorative. The abraded particles may cause decreased transparency due to light scattering and may impart an unnatural color. Draiswerke, Inc., Mahwah, N.J., has applied a polyurethane coating on the agitator and grinding chamber for their PML-H/V machine. The pigment from this coating, however, also contaminates the composites, making them unacceptable for dental use.
The predominant types of milling methods are dry milling and wet milling. In dry milling, air or an inert gas is used to keep particles in suspension. However, fine particles tend to agglomerate in response to van der Waals forces which limits the capabilities of dry milling for submicron particle sizes. Wet milling uses a liquid such as water or alcohol to control reagglomeration of fine particles. Therefore, wet milling is typically used for comminution of submicron-sized particles.
A wet mill typically includes spherical media that apply sufficient force to break particles that are suspended in a liquid medium. Milling devices are categorized by the method used to impart motion to the media. The motion imparted to wet ball mills includes tumbling, vibratory, planetary and agitation. While it is possible to form submicron particles with each of these types of mills, the agitation or agitator ball mill is typically most efficient.
The agitator ball mill, also known as an attrition or stirred mill, has several advantages including high energy efficiency, high solids handling, narrow size distribution of the product output, and the ability to produce homogeneous slurries. The major variables in using an agitator ball mill are agitator speed, suspension flow rate, residence time, slurry viscosity, solid size of the in-feed, milling media size and desired product size. As a general rule, agitator mills typically grind particles to a mean particle size approximately 1/1000 of the size of the milling media in the most efficient operation. In order to obtain mean particle sizes on the order of 0.05 .mu.m to 0.5 .mu.m, milling media having a size of less than 0.45 mm can be used. Milling media having diameters of about 0.2 mm and about 0.6 mm are available from Tosoh Ceramics, Bound Brook, N.J. Thus, to optimize milling, it is desirable to use a milling media approximately 1000 times the size of the desired particle. This minimizes the time required for milling.
As discussed briefly above, the use of known milling processes to achieve such fine particle sizes was difficult due to contamination of the slurry by the internal components of the mill. Further contamination has also been introduced into the slurry by abrasion and spalling of the milling media. By using hard materials for the milling media, such as yttria stabilized zirconia (YTZ or Y-TZP, where TZP is tetragonal zirconia polycrystal), which has a Vickers hardness of equal or greater than about 11 GPa, the contamination by spalling from the milling media and abrasion from the mill is minimized, thereby minimizing opacification of the filler. Y-TZP has a fine grain, high strength and a high fracture toughness. High strength Y-TZP is formed by sintering at temperatures of about 1550.degree. C. to form tetragonal grains having 1-2 .mu.m tetragonal grains mixed with 4-8 .mu.m cubic grains and high strength (1000 MPa), high fracture toughness (8.5 MPa m.sup.1/2) and excellent wear resistance. Due to its resistance to abrasion and spalling, the use of Y-TZP provides a suitable milling media for providing relatively pure, highly translucent structural fillers having mean particle sizes less than 0.5 .mu.m. The YTZ milling media, however, is very expensive. So although low contamination agitator milling with YTZ milling media is time efficient, it is costly due to the expense of the milling media as well as the cost of the machine.
Furthermore, despite some reduction in contamination of the ground filler particulate by the use of the abrasion-resistant YTZ milling media, agitator ball mills still introduce an unacceptably high level of contamination into dental composites containing the ground filler. The high intensity of the grinding action produced by the agitator, and the high momentum of the media, result in abrasion and spalling of the grinding chamber wall, as discussed above. One proposed solution is described in Applicants' copending U.S. patent application Ser. No. 09/271,639 filed Mar. 17, 1999, and entitled "Agitator Mill and Method of Use for Low Contamination Grinding," which is incorporated by reference herein in its entirety. It is proposed that various internal components of the mill be fabricated from the abrasion-resistant YTZ material and that the interior of the grinding chamber be coated with a non-pigmented polymer that would not negatively affect the optical properties of the ground material.
Another possible solution to the problem of contamination in ground dental fillers is to use the less harsh method of vibratory milling. Vibratory ball mills are often used for submicron particle grinding because they provide a high production rate at low capital cost, fine and uniform product size distribution, low power consumption, and low contamination. The rate of milling is a function of the shape and size of the media. Cylindrical media are generally preferred, according to Engineered Materials Handbook.RTM., Desk Edition, ASM International, p 742 (1995), because they spin on an axis and therefore produce small shear forces. The major variables in using a vibratory mill are the amplitude of vibration, energy developed in the mill, slurry viscosity, solid size of the in-feed, milling media size and desired product size. Because vibratory milling involves low intensity grinding, abrasion and spalling of the grinding chamber wall and the milling media are less of a concern, as compared to agitator mills. Even this low intensity grinding, however, may still contribute to an unacceptable level of contamination in the ground filler.
Furthermore, the vibratory mill has not previously been found useful for low contamination grinding of particles having an average size less than the wavelength of visible light. U.S. Pat. No. 4,544,359 describes a dental restorative material with a borosilicate glass/barium silicate filler having an average particle size diameter of from about 0.5 .mu.m to 5.0 .mu.m. The filler is ground by a conventional wet milling process, such as vibratory milling, using a grinding or milling media such as low alumina, porcelain balls, stainless steel balls, borosilicate glass rods, or any other low alumina, non-contaminating grinding medium. Thus, there is a need for a low contamination, vibratory grinding mill and method of use to produce particles having a mean particle size of less than 0.5 .mu.m, where the filler does not opacify upon exposure to visible light.